Aimed at the present traditional air leakage detecting method, the direct pressure or internal differential pressure method had a drawback that the detection accuracy was easily affected by the leakage of cutoff valve and pipeline, a highpressure valve air leakage detecting system based on the external pressure differential pressure method is put forward. The hardware and software system of the testing device was designed on the basis of the detection scheme, and the experiment was carried out.Experimental results show that as compared with the system by the direct pressure method and the internal pressure differential pressure method, this system is effective in reducing the impact caused by the air leakage of cutoff valve and gas pipeline and other external factors on the accuracy of detection. This detection system has good stability and short response time, and can be applied to the online batch testing of the factory.